The present invention relates to a golf ball having high rebound characteristics and exceptional controllability at approach shot from the rough, which accomplished by controlling its cover material morphology.
There have been various requirements for physical properties of golf balls, such as flight distance, controllability, shot feel, click sound and the like. Among these, the controllability is rated by top-rank golfers as one of the most important properties for scoring; especially when approach shots are made from the rough. As such, a technique used to improve controllability involves making the cover softer, which increases the amount of spin, even when approach shots are made from the rough. However, golf balls obtained using such a technique suffer from the drawbacks of diminished rebound characteristics and reduced flight distance.
A main object of the present invention is to provide a golf ball which has high rebound characteristics and exceptional controllability at approach shot from the rough.
According to the present invention, the object described above has been accomplished by forming a base resin of a cover from a material having at least two phases, and setting the phase angle difference of these phases, as measured by an atomic force microscope to within a specific range to control the morphology in such a way that the phase that is contained in the base resin of the cover, which has the maximum phase angle, is present in the form of a continuous matrix, and the phase with the minimum phase angle is present in discrete locations within the matrix, thereby providing a golf ball having high rebound characteristics and exceptional controllability at approach shot from the rough, even when hit through grass or when wet conditions tend to limit the amount of spin when approach shots are made from the rough.
The present invention relates to a golf ball comprising a core and a cover covering the core, wherein the base resin of the cover is formed from a material having at least two phases whose phase angle difference is at least 2 degrees as measured by using an atomic force microscope; a maximum phase angle phase, which is contained in the base resin of the cover and has an absolute value of a maximum phase angle, is present in the form of a continuous matrix; and a minimum phase angle phase, which has an absolute value of a minimum phase angle, is present in discrete locations within the matrix.
In order to put the present invention into a more suitable practical application, it is desired that the minimum phase angle phase have a mean dispersion particle size of no more than 700 nm.
It is universally accepted that the less hardness a golf ball cover material has, the more spin will result; however, it goes without saying that low hardness also causes a decrease in rebound characteristics and durability. Using a hard material, accordingly, in order to obtain an increase in the rebound characteristics and durability, translates into diminished spin performance. Upon conducting diligent research on the cover material, the inventors of the present invention discovered that by fabricating the cover from a specific material, and also by using such materials in combination, will result in an increase in both the rebound characteristics and the durability. The inventors predicted that the appearance of these properties was related to the phase structure (morphology) of the material, and by using an atomic force microscope to analyze the morphology, which is not observable using common electronic microscopes, it was discovered that the cover contains a so-called xe2x80x9csea-islandxe2x80x9d structure, wherein the maximum phase angle phase, which contains the largest phase angle in the material used in the base resin of the cover, is present in a continuous matrix (xe2x80x9cseaxe2x80x9d), and the minimum phase angle phase, which contains the smallest phase angle, is present in discrete locations (xe2x80x9cislandsxe2x80x9d) within the xe2x80x9cseaxe2x80x9d, and in the cover there exists a very strong correlation between the phase angle difference of the minimum phase angle phase and the maximum phase angle phase in the material used in the base resin of the cover, the amount of spin and the rebounding performance. Moreover, it is preferable for the mean dispersion particle size in the minimum phase angle phase (xe2x80x9cislandsxe2x80x9d) to be no greater than 700 nm.
In the detailed description of the present invention which follows, the golf ball of the present invention comprises a core and a cover formed on said core. The core used in the golf ball of the present invention may be e.g. a solid core (of a single layer structure or one comprising two or more layers) or a thread-wound core. Examples of materials which can be used as a solid core include thermoplastic elastomers, thermoplastic resins or rubber compositions which comprise a base rubber, an organic peroxide, a co-crosslinking agent and the like. Of these, the rubber compositions are preferable.
In solid cores as described above, the base rubber may be natural and/or synthetic rubber, which has been conventionally used for solid golf balls. Preferred is so-called high-cis polybutadiene rubber containing a cis-1,4-structure of not less than 40%, preferably not less than 80% and even more preferably not less than 90%. According to need, the high-cis polybutadiene rubber can be mixed with natural rubber, polyisoprene rubber, styrene butadiene rubber or ethylene-propylene-diene rubber (EPDM) and the like in amount of 0 to 50 parts by weight, based on 100 parts by weight of the base rubber.
Examples of co-crosslinking agents include xcex1,xcex2-unsaturated carboxylic acids with 3 to 8 carbon atoms (e.g. acrylic acid, methacrylic acid, etc.), a mono- or bivalent metal salt such as the zinc or magnesium salt thereof, functional monomers such as triethanolpropane trimethacrylate and mixtures thereof. The preferred co-crosslinking agent is zinc acrylate because it imparts high rebound characteristics to the resulting golf ball. The amount of the co-crosslinking agent is 5 to 50 parts by weight, and preferably 10 to 40 parts by weight, based on 100 parts by weight of the base rubber. When the amount is larger than 50 parts by weight, the core is too hard, and the shot feel is poor. On the other hand, when the amount is smaller than 5 parts by weight, an appropriate level of hardness is not obtained, and the rebound characteristics are severely degraded, which reduces the flight distance.
Examples of organic peroxides include dicumyl peroxide, 1,1-bis(t-butylperoxy)-3,3,5-trimethylcyclohexane, 2,5-dimethyl-2,5-di(t-butylperoxy)hexane, and di-t-butyl peroxide and the like. The preferred organic peroxide is dicumyl peroxide because it imparts high rebound characteristics to the resulting golf ball. The amount of the organic peroxide is 0.1 to 5.0 parts by weight, and preferably 0.2 to 3.0 parts by weight, based on 100 parts by weight of the base rubber. When the amount is smaller than 0.1 parts by weight, the core becomes too soft, the rebound characteristics are degraded, which reduces the flight distance. On the other hand, the amount is larger than 5.0 parts by weight, the core becomes too hard, which causes cracks to form.
Where appropriate, it is possible to compound a component which is typically used in the manufacture of solid golf ball cores together with the rubber composition; e.g., fillers such as zinc oxide, barium sulfate and the like as a specific gravity regulator, and other additives such as antioxidants, peptizing agents and organic sulfide compounds. Preferably the amount of the filler is 5 to 50 parts by weight, the amount of the antioxidant is 0.1 to 5 parts by weight, the amount of the peptizing agent is 0.1 to 5 and the amount of the organic sulfide compound is 0.1 to 2 parts by weight, based on 100 parts by weight of the base rubber.
A method for manufacturing the core when a rubber composition is used involves the rubber composition to be used in the core being mixed and then press-molded under applied heat of e.g. 140 to 165xc2x0 C. for 10 to 40 minutes in a mold with upper/lower sections containing hemispherical cavities, and when a thermoplastic resin is used, involves injection molding the composition for the core using a similar mold. Multi-piece solid cores containing at least two layers can be obtained by forming the compositions for the core in a laminar structure. A method for manufacturing multi-piece solid cores when a rubber composition is used involves the composition for the core first being molded into a semi-vulcanized hemispherical half-shell, then using two half-shell to wrap a single layer structure core as mentioned in the foregoing, and molding the whole under applied pressure at 130 to 180xc2x0 C. for 5 to 60 minutes, and when a thermoplastic resin is used, involves the single layer structure core becoming encased in the composition for the core as described in the foregoing by injection molding the composition directly thereon. The surface of the resulting core may be buff-ground in order to enhance the tight adhesion between it and the cover. In order to enhance the tight adhesion between the cover and multi-piece solid cores, as well as between the layers of the core, the surface of the core may be buff-ground prior to having it be covered with the adjacent outer layer.
In thread-wound cores as described above, conventionally used thread-wound cores are suitable for use, with the core comprising a center and a thread rubber layer formed by winding a thread rubber in an elongated state around the center. The center may be liquid-based (xe2x80x9cliquid centerxe2x80x9d) or rubber-based (xe2x80x9csolid centerxe2x80x9d). The thread rubber used for winding around the center may be of the same kind which is conventionally used in thread-wound layers in thread-wound golf balls; e.g., it can be obtained by vulcanizing a rubber composition in which natural rubber or natural rubber and synthetic polyisoprene have been compounded with sulfur, a vulcanization aid, vulcanization accelerator, antioxidant and the like. A thread-wound core can be produced by drawing the thread rubber about 1000% over the center and winding it there. However, such solid and thread-wound cores are given by way of illustrative examples only, and the invention shall not be limited thereto.
The diameter of the core is 36.8 to 41.2 mm, and preferably 37.8 to 40.8 mm. When the core diameter is smaller than 36.8 mm, the cover will become too thick, which will cause the rebound characteristics to degrade, owing to the increasing volume fraction of the cover component. On the other hand, when the diameter is larger than 41.2 mm, the cover will become too thin, and the durability is poor.
Next, the core is covered with a cover. It is required that the base resin of the cover of the present invention comprise a material which has at least two phases in which the phase angle difference as measured by an atomic force microscope is at least 2xc2x0, preferably 2 to 45xc2x0, and even more preferably 2 to 20xc2x0, and have a xe2x80x9csea-islandxe2x80x9d phase structure, in which the maximum phase angle phase, which contains the largest phase angle in the material used in the base resin of the cover, is present in a continuous matrix (xe2x80x9cseaxe2x80x9d), and the minimum phase angle phase, which contains the smallest phase angle, is present in discrete locations (xe2x80x9cislandsxe2x80x9d) within the xe2x80x9cseaxe2x80x9d. If the phase angle difference as measured by an atomic force microscope between the two or more phases in the cover material is smaller than 2xc2x0, the strength in the direction of shear will be insufficient, and particularly when an approach shot is made from the rough, there will be reduced backspin, leading to poor spin performance. Moreover, if the phase structure of the cover assumes a connected phase as opposed to a xe2x80x9csea-islandxe2x80x9d structure, the cover will become hard, and, particularly when an approach shot is made from the rough, there will be reduced backspin amount, leading to poor spin performance. Conversely, the more the phase angle difference exceeds 45xc2x0, the lower the cover durability will be. Also, if the minimum phase angle phase, which contains the smallest phase angle, is present in a continuous matrix (xe2x80x9cseaxe2x80x9d), and the maximum phase angle phase, which contains the largest phase angle, is present in discrete locations (xe2x80x9cislandsxe2x80x9d) within the xe2x80x9cseaxe2x80x9d, the cover will become too hard, and particularly when an approach shot is made from the rough, the spin amount will be reduced, leading to poor controllability. It is moreover desirable for the minimum phase angle phase in the cover used in the golf ball of the present invention to have a mean dispersion particle size of no more than 700 nm, preferably of no more than 600 nm, even more preferably of no more than 500 nm and even still more preferably of no more than 200 nm. If the mean dispersion particle size is larger than 700 nm, the cover will become hard, and particularly when an approach shot is made from the rough, the backspin amount will be reduced, leading to poor spin performance. It is preferable for the mean dispersion particle size to be as small as possible; however, it is desirable for it to remain at least 1 nm, preferably at least 10 nm and even more preferably at least 50 nm.
The phrase, xe2x80x9cthe phase angle as measured by an atomic force microscopexe2x80x9d as used herein shall refer to the fact that a model SPI-300HV atomic force microscope manufactured by Seiko Instruments inc. is used, that a cantilever with a 40 N/m spring constant, to which a probe has been attached, is used in 200 to 350 Hz frequency conditions, and that the phase angle is measured in xe2x80x9cphase modexe2x80x9d, which is the mode for measuring phase angles. In other words, the probe which is attached to the cantilever, which vibrates at the frequencies, is brought into contact with the samples, and the response is detected using reflected laser light. The energy loss will be greater in the soft areas of the sample, and as such the phase delay; i.e., the phase angle, will be greater. By scanning the sample surface while the cantilever is made to vibrate, an image can be obtained whereby the sample surface is color-coded according to phase angle size. The dispersion particle size of the minimum phase angle phase can be determined from this image.
It is desirable for the cover of the present invention to have a thickness of 0.84 to 3.00 mm and preferably of 1.0 to 2.5 mm. When the thickness of the cover is smaller than 0.84 mm, then the cover will be less durable and become damaged, while when its thickness is larger than 3.00 mm, the rebound characteristics will be degraded, or the shot feel will be hard and thereby not as good.
Examples of materials to be used in the base resin in the cover of the present invention, which will form the minimum phase angle phase, include so-called ionomer resins, in which at least a portion of the carboxyl groups in a copolymer of an xcex1-olefin and an xcex1,xcex2-unsaturated carboxylic acid have been neutralized with metal ions. Examples of the xcex1-olefins contained in the ionomer resin preferably include ethylene and propylene, while examples of the xcex1,xcex2-unsaturated carboxylic acid preferably include acrylic acid and methacrylic acid. Examples of the neutralizing metal ions include alkaline metal ions; e.g. sodium, potassium and lithium ions, bivalent metal ions; e.g., zinc, calcium and magnesium ions, trivalent metal ions; e.g., aluminum and neodymium ions, and mixtures thereof. However, among these, sodium, zinc and lithium ions are preferably used due to their rebound characteristics and durability. Specific examples of the ionomer resin are not limited to the above; Hi-milan 1555, 1557, 1605, 1706, 1707, AM7315 and AM 7317 (Mitsui Du Pont Polychemical Co., Ltd.), Surlyn 7930, 8511, 8512 (Du Pont Co.), and Iotek 7010 and 8000 (Exxon Chemical Co.) can all be given as examples. The ionomers may each be used alone or in combinations of two or more.
Examples of materials to be used in the base resin in the cover of the present invention, which will form the maximum phase angle phase, include thermoplastic resins or thermoplastic elastomers. Specific examples are not limited thereto; other examples include styrene-based thermoplastic elastomers commercially available from Asahi Chemical Industry Co., Ltd., under the trade name xe2x80x9cTuftecxe2x80x9d (e.g., Tuftec H1051); polyester-based thermoplastic elastomers commercially available from Toray-Du Pont Co., Ltd., under the trade name xe2x80x9cHytrelxe2x80x9d (e.g., Hytrel 3548 and Hytrel 4047); polyamide-based thermoplastic elastomers commercially available from Toray Co., Ltd., under the trade name xe2x80x9cPebaxxe2x80x9d (e.g., Pebax 4033SN); polyurethane-based thermoplastic elastomers commercially available from Dainippon Ink and Chemicals Inc., under the trade name xe2x80x9cPandexxe2x80x9d (e.g., Pandex T-7298); and polyurethane-based thermoplastic elastomers commercially available from Takeda Badische Urethane Industries, Ltd., under the trade name xe2x80x9cElastollanxe2x80x9d (e.g., Elastollan ET890).
However, the thermoplastic elastomers have a morphology whereby the soft rubber portion and the hard crystalline portion are mixed together, and therefore the cover of the present invention can be constituted from them alone. In other words, the thermoplastic elastomers have a phase structure whereby the soft rubber portion, which has the largest phase angle, is present in a continuous matrix (xe2x80x9cseaxe2x80x9d), and the hard crystalline portion, which has the smallest phase angle, is present in discrete locations (xe2x80x9cislandsxe2x80x9d) within the matrix, and the difference between the two phase angles is at least 2xc2x0.
Other than the base resin, any of various additives; e.g., pigments such as titanium dioxide, dispersants, antioxidants, UV absorbers, photostabilizers etc. may be added as needed to the cover of the present invention.
In other words, the present invention specifically comprises the following three embodiments.
(i) a golf ball comprising a core and a cover which has been applied to said core, wherein said cover comprises a base resin which comprises an ionomer resin and a thermoplastic resin and which comprises a material which contains at least two phases in which the phase angle difference as measured by an atomic force microscope is at least 2xc2x0; the maximum phase angle phase, which contains the largest phase angle, and which is contained in the base resin of said cover, being present in a continuous matrix, and the minimum phase angle phase, which contains the smallest phase angle, being present in discrete locations in said matrix.
(ii) a golf ball comprising a core and a cover which has been applied to said core, wherein said cover comprises a base resin which comprises an ionomer resin and a thermoplastic elastomer and which comprises a material which contains at least two phases in which the phase angle difference as measured by an atomic force microscope is at least 2xc2x0; the maximum phase angle phase, which has the largest phase angle, and which is contained in the base resin of said cover, being present in a continuous matrix, and the minimum phase angle phase, which has the smallest phase angle, being present in discrete locations in said matrix.
(iii) a golf ball comprising a core and a cover which has been applied to said core, wherein said cover comprises a base resin, which comprises a thermoplastic elastomer and which comprises a material which contains at least two phases in which the phase angle difference as assessed by an atomic force microscope is at least 2xc2x0; the maximum phase angle phase, which has the largest phase angle, and which is contained in the base resin of said cover, being present in a continuous matrix, and the minimum phase angle phase, which has the smallest phase angle, being present in discrete locations in said matrix.
In the embodiment (i), a thermoplastic resin is used as a material which forms the maximum phase angle phase, and an ionomer resin is used as a material which forms the minimum phase angle phase, with the thermoplastic resin thought to form the xe2x80x9cseaxe2x80x9d portion, and the ionomer resin thought to constitute the xe2x80x9cislandxe2x80x9d portions.
In the embodiment (ii), the thermoplastic elastomer alone provides the xe2x80x9csea-islandxe2x80x9d structure, which is normal, as mentioned in the foregoing; however, it is thought here that the thermoplastic elastomer forms the xe2x80x9cseaxe2x80x9d, containing the large phase angle, and therein the ionomer resin forms the relatively harder xe2x80x9cislandxe2x80x9d portions. Consequently, it is necessary in this embodiment for the phase angle difference between the maximum phase angle phase and the minimum phase angle phase in such a material to be at least 2xc2x0.
In the embodiment (iii), the structure is constituted solely by the thermoplastic elastomer, and, as mentioned in the foregoing, the soft rubber portion and the hard crystalline portion are mixed together in the thermoplastic elastomer, and it is therefore thought that the soft portion forms the xe2x80x9cseaxe2x80x9d portion, while the hard portion constitutes the xe2x80x9cislandxe2x80x9d portions.
It is moreover possible to obtain the desired phase structure by adjusting not only the type of structural component in the maximum phase angle phase and minimum phase angle phase of the material used in the base resin of the cover, but also e.g. the viscosity, volume fraction, temperature during kneading and shear strength during kneading of both aforesaid phases. In other words, if the types of structural components of the cover base resin material are the same, then by changing the other aforesaid factors it is possible to obtain a phase structure whereby the structures of the maximum phase angle phase and the minimum phase angle phase are reversed.
A method of covering on the core with the cover is not specifically limited, but may be a conventional method. For example, there can be used a method comprising molding the cover composition into a hemispherical half-shell in advance, covering the core with the two half-shells, followed by pressure molding at 130 to 170xc2x0 C. for 1 to 5 minutes, or a method comprising injection molding the cover composition directly on the core to cover it. At the time of molding the cover, dimples may be optionally formed on the surface of the golf ball. Furthermore, paint finishing or marking with a stamp may be optionally provided after the cover is molded. The multi-piece solid golf ball of the present invention is formed to a diameter of at least 42.67 mm and a weight of no more than 45.93 g, in accordance with the regulations for golf balls.
The cover of the present invention may have a single layer structure, or a multi-layered structure comprising two or more layers; in the event that a multi-layered cover structure is adopted, the outermost layer should satisfy the phase structure and properties as described in the foregoing.
Through controlling the cover material morphology, it is possible with the present invention to provide a golf ball which has high rebound characteristics and exceptional controllability, even when approach shots are made from the rough.